Social capital has been defined in numerous ways and can refer to sociability, social networks and social support, trust, reciprocity and community and civic engagement. The effect of social capital upon children's well-being is under-researched. A theoretical model identifies potential linkages between social capital, intermediate variables and child well-being. After presenting some of the key issues in the measurement of social capital (the separation of cognitive and structural social capital, the exclusion of outcomes of social capital, the definitions of community) this paper then considers the limited literature on conceptualizing social capital in relation to children. Quantitative measures of children's social capital need to build upon the qualitative work available. An ongoing longitudinal study covering 8000 children in four developing countries (Vietnam, Peru, Ethiopia, and India) is introduced as one way forward.